Edward Hopper is one of those painters who are always there on the edge of your awareness. His famous canvases are constantly being reproduced. But when you come up against the real things, you discover that his work is even stranger and more haunted--more impregnable--than you remembered. That's the lesson of the mesmerizing Hopper show that has opened at the Boston Museum of Fine Arts, where it remains until Aug. 19 before traveling to Washington and Chicago. That's also the Hopper paradox. He's the easy-to-read artist who's always just beyond our grasp.